PROJECT ABSTRACT / SUMMARY More than 30 years have passed since the discovery of the ubiquitin-proteasome system, yet we still lack comprehensive insights into its regulatory mechanisms. This limitation arises because most of the approximately 600 ubiquitin ligase enzymes in mammals remain largely uncharacterized. Our laboratory has focused on how Cullin-RING Ubiquitin Ligase (CRL) complexes control the three essential dimensions of cellular life: proliferation, survival, and differentiation. We have played a central role in elucidating how, when, where and, most importantly, why CRLs mediate the degradation of key cellular regulators. The research in my laboratory initially focused on the paradigm of the timed regulation of the mammalian cell cycle by the ubiquitin-proteasome system, which we established a number of years ago. We have since expanded our research into five fields of study: (i) cell signaling, (ii) cell cycle control, (iii) the DNA damage response, (iv) oncology, and (v) the circadian clock. This expansion of our interests has been possible thanks to our discovery-driven approach that, in contrast to the more common hypothesis-driven approach, is unbiased. Yet, the results of genetics and proteomics screens are not our end goal, but they guide us to explain the underlying biological concepts and mechanisms. Importantly, they often lead us to unexpected and unexplored new territories, opening our horizons. In summary, we use a comprehensive and interdisciplinary approach to break new grounds and make transformative discoveries that will provide new mechanistic insights into fundamental biological processes, particularly those processes that are regulated by CRLs. The current proposed research will cover all the mechanistic studies performed in our laboratory concerning areas i-iii listed above. In particular, we will leverage our recognized expertise in the ubiquitin field to discover molecular mechanisms by which CRLs control signal transduction pathways, autophagy, cell cycle progression, and DNA repair. Moreover, we will perform biochemical, cellular and in vivo analyses to illuminate how CRLs are regulated to function as switches between diverse cell fates. These findings will substantially advance our understanding of the proper execution of crucial cellular processes, potentially shedding light on the etiologies and treatments of human diseases.